The present invention relates to feed-through filter capacitor assemblies, and more particularly to the conductive adhesive connection between the ceramic capacitor and the conductive pin passing through a feed-through passage in the capacitor.
Feed-through filters are utilized to separate unwanted interference from a signal path, for example in connection with implantable medical devices, such as heart pacemakers and the like. These electronic devices are often constructed having an outer housing in which the necessary electronic components are contained. For implantable devices, the outer housing must be formed of a material that is biocompatible and capable of shielding the electronics within the housing from external sources of electromagnetic interference (EMI). Titanium is often used to satisfy these dual requirements of shielding and biocompatibility. A conductive wire or pin extends from the electronics within the outer housing to a desired external location, such as one inside the body. The conductive wire may act as an antenna picking up spurious radio frequency signals that interfere with proper operation of the device, such that a filtering capacitor is desirable. A small capacitor is fitted in an annular space formed between a conductive housing (ferrule) and the conductive pin. The capacitor is formed from a dielectric ceramic and two sets of electrode plates, wherein one set of plates is electrically connected to the conductive pin and the other set of plates is electrically connected with the conductive housing. For some applications, however, there is no outer housing, such as with a connector cable. Regardless, the electrical connection between the conductive pin and the capacitor electrode plates is generally achieved by means of a solder, an electrically conductive adhesive material or brazing. The use of an organic polymeric-based conductive adhesive is preferable, but in use, cracking in the capacitor body has been observed. Because polymeric materials exhibit shrinkage during curing, tension is placed on the ceramic body in the area of the feed-through passage. That tension, if sufficiently high, can crack the capacitor body.
Thus, there is a need to enable the use of conductive adhesives for electrically connecting the capacitor with the conductive pin in a fccd-through filter capacitor assembly without cracking the capacitor body during curing.
100041 The present invention provides a feed-through filter capacitor assembly in which an electrically conductive adhesive modified with a filler of low coefficient of thermal expansion (CTE) is used to provide the conductive relation between the conductive pin and the electrode plates of the ceramic capacitor. A feed-through filter capacitor has a ceramic body and first and second sets of electrode plates. At least one internal passageway is formed through the ceramic body and a conductive pin passes through each passageway in conductive connection with the first set of electrode plates. An electrically conductive adhesive in the passageway provides the conductive connection. The conductive adhesive comprises an organic polymer-based adhesive component that has a CTE greater than the CTE of the capacitor ceramic body and a conductive metal filler to provide the electrical connection between the conductive pin and the electrode plates. In accordance with the present invention, the conductive adhesive is further provided with a CTE-lowering filler that has a CTE lower than the CTE of the conductive metal filler. This low CTE filler effectively lowers the overall CTE of the adhesive to a value closer to the CTE of the capacitor ceramic body. By lowering the CTE of the adhesive, less shrinkage occurs during curing, and therefore, less tension is placed on the capacitor body, thereby reducing the likelihood of cracking the capacitor body.